Effect of planarization of the bottom superconducting yttrium-barium-copper-oxide layer in the multilayer structure

Abstract

YBCO/YSZ/Si02/YSZ/YBCO multi-layer structures have been successfully grown on single crystal YSZ substrates. The YBCO superconducting layers (300 nm thick) were deposited using pulsed laser deposition (PLD). The YSZ layers (300 nm thick) which are biaxially aligned were deposited using PLD and the ion beam assisted deposition (IBAD). A thick silicon dioxide layer (2-4 microns) was sandwiched between the YSZ layers to meet the low dielectric constant requirement for multi-chip module applications. However, if the bottom superconducting layer was patterned into interconnecting lines as required in device applications, the surface of the YSZISi02NSZ on top of the patterned bottom superconducting layer had a roughness of about 500 nm. As a result, the top YBCO was no longer superconducting. Thus, planarization of the patterned bottom superconducting layer becomes a key issue. We have developed a "fill-in and lift-off' process to fill the gap between the patterned bottom superconducting lines with YSZ. As a result, we were able to reduce the surface roughness of the bottom YBCO layer to about 10 nm so the top layer was superconducting with a critical temperature of 87 K. 2pm) whose dielectric constant is less than four was chosen to isolate the two superconducting interconnects, and two biaxially aligned Yttrium Stabilized Zirconia (J'SZ) layers were deposited on top and underneath the SiOl, as b u f k layers to prevent diffusion of the amorphous SiO, into the YBCO layers. However, when the bottom YBCO layer is patterned into interconnecting lines, the surface of the YSZ/SiO,/YSZ is no longer flat. The roughness of the surface is usually greater than the thickness of the bottom YBCO layer due to over etching in the patterning process. Since the top YBCO layer is found not to be superconducting when fabricated on a rough surface, the planarization of such a rough surface becomes a key issue. In this paper we report on a "fill-in and lift-off' planarization procedure and the effect of planarization of the bottom superconducting YBCO layer on the performance of the top YBCO layer superconducting layer in the multi-layer structure. EXPERIMENTAL DETAILS AND RESULTS